Gold coating by salt decomposition

ABSTRACT

Nickel substrate is provided with a thin gold coating or plating by depositing a mixture of potassium gold cyanide and petrolatum on the surface and heating the surface to a temperature sufficient to cause decomposition of the potassium gold cyanide and removal of the petrolatum.

United Stats Pate [151 3,645,774 5] Feb. 29, 1972 Schreck 1541 GOLD COAT-ENG BYSALT DECOMPOSITION [72] Inventor: John Thomas Schreck, Mechanicsburg,

[73] Assignee: AMP Incorporated, Harrisburg, Pa.

[22] Filed: Sept. 15, 1969 21 App]. N0.I 858,061-

52 us. c1. ..117/46 CA, 117/130 R, 117/227, 106/272 51 lm. c1. ..B44d 1/44, C236 17/02 [58] Field 61 Search ..1 l7/46 CB, 46 cc, 130 R, 227, 1 17/46 at [56] References Cited UNITED sTATEs PATENTS 2,429,088 10/1947 Box ..117/227 2,979,424 4/1961 Whitehurst et a]. ..117/46 cc 3,214,292 10/1965 EdSOll ..117/227 3,266,929 8/l966 Lareau ..117/227 3,313,632- 4/l967 Langleyetal ..ll7/l30R 3,320,293 5/1967 Coffey ..ll7/l30R 10/1968 Ballard ..1 17/227 Primary Examiner-William D. Martin Assistant Examiner-M. Sofocleous Att0rney- -Curtis, Morris and Safford, Marshall M. Holcombe, William l-lintze, William J. Keating, Frederick W. Raring, John R. Hopkins, Adrian J. La Rue, Jay L. Seitchik and Ronald D. Grefe [5 7] ABSTRACT Nickel substrate is provided with a thin gold coating or plating by depositing a mixture of potassium gold cyanide and petrolatum on the surface and heating the surface to a temperature sufficient to cause decomposition of the potassium gold cyanide and removal of the petrolatum.

3 Claims, 1 Drawing Figure GOLD COATING BY SALT DECOMPOSITION BACKGROUND OF THE INVENTION Gold platings are ordinarily provided on metal articles, for either functional or decorative purposes, by electrolysis of any one of several commercially available gold salts. In the electronic industry, it is common practice to provide extremely thin gold platings on contact surfaces because of the virtual immunity of gold to corrosion and becauseof its excellent electrical conductivity. Thus, contact terminals are quire often provided with gold platings having a thickness of from 50 to 100 millionths of an inch in order to improve the initial performance of the terminal and to ensure them a long and trouble free life. Gold platings are provided on most extremely high quality and extensive terminals and are also provided on vast numbers of terminals of intermediate quality. The plating costs for terminals in the latter price range constitutes a significant portion ofthe overall cost of the terminal.

Where gold is deposited on a contact terminal in order to achieve enhanced contact properties, the gold is required only at the actual contact area and need not be provided on the entire surface of the terminal. In most plating processes, however, the entire part being plated must be immersed in the electrolyte so that a plating of uniform thickness is provided over the entire surface of the part being plated. It follows that a substantial reduction in plating costs in contact terminals or similar articles could be achieved if an efficient and inexpensive method could be employed to deposit gold selectively in the precise area where it is required.

An object of the instant invention is to provide an improved method of plating or coating gold onto a metal substrate. A further object is to provide an improved method of plating gold by decomposition ofa gold salt. A still further object is to provide a composition of matter which is effective to provide a plating on a nickel substrate by merely coating the composition onto the surface and heating the surface to a temperature sufficient to cause decomposition of the composition and deposition of gold on the substrate surface.

These and other objects of the invention are achieved in a preferred embodiment which is briefly described in the foregoing abstract, which is described in detail below, and which is illustrated by the accompanying single sheet of drawing which shows a typical contact terminal and shows the manner of providing a gold plating on selected portions on surfaces thereof.

In accordance with a preferred embodiment of the instant invention, potassium gold cyanide KAu(CN) is mixed with a suitable vehicle, preferably a hydrocarbon such as a grease or paraffin, and a coating of the mixture is applied to the surface being plated. The surface is then heated to a temperature of about 450 F. to 520 F. to cause elimination of the vehicle by combustion and/or volatilization, decomposition of the potassium gold cyanide, and deposition of the gold value ofthe gold cyanide on the surface of the part being plated. The parts being plated may be quenched in water immediately after heating in order to remove any carbonaceous residue from the vehicle although such residue can be removed by other means as by brushing.

The petrolatum-gold salt mixture may be applied to the surface in any manner as by brushing, spraying, or by application with a nozzle extending from a pressurized container. The thickness of the gold cyanide-petrolatum mixture should be relatively uniform over the entire area being plated or coated with metallic gold in order to achieve a uniform plating thickness in the finished plated article. In this respect, it should be noted that the heating of the part to the decomposition temperature for the potassium gold cyanide will, to some extent, cause flow of the petrolatum or other vehicle of the mixture so that the thickness of the layer will be fairly uniform for decomposition of the gold salt and removal for destruction of the vehicle'takes place. In practice, good results have been achieved by depositing a coating having a thickness of about 0.03 to 0.06 inches on the surface of the part being plated.

The ratio of the gold cyanide to the vehicle may vary within relatively wide limits. For example, good results have been obtained with mixtures containing one part by weight of gold cyanide and five parts by weight of petrolatum. Good results have also been obtained with mixtures containing equal parts by weight of gold cyanide and petrolatum. It has been observed that as the concentration of gold cyanide in the mixtures increased, the plating thickness in the finished plated article is also increased if the thickness of the coating applied to the surface remains constant. The thickness of the plating can therefore be controlled to a large extent by merely varying the amount of gold cyanide-petrolatum mixture. It has been observed that where equal parts by weight of gold cyanide and petrolatum are employed and the thickness of the coating of the mixture applied to the part is about 0.03 inches, a gold plating thickness of 200 microinches will be obtained. Platings in the thickness range of about 50 to 200 microinches are commonly used in the electrical industry for contact terminals so that plating thickness can be controlled within the operative range where platings are being applied in accordance with the invention. It should be added that the amount of gold in the mixture can exceed the 50-50 ratio noted above although there is little need for platings thicker than about 200 microinches for contact terminals.

The drawing FIGURE illustrates the manner in which selected surface portions of a contact terminal can be pro vided with a thin gold plating or coating of gold in accordance with the invention. The terminal 2 shown has a cylindrical end portion 4 of reduced diameter extending from an intermediate body portion 6, the rearward end 8 of the terminal being crimped onto a wire 10. The reduced diameter tip 4 of the terminal is adapted to be received in a contact socket and should be provided with a gold plating for best results although gold is not required on the remainder of the terminal for contact purposes. In the terminal shown, the end 14 of the cylindrical portion 12 can be gold plated by application of the potassium gold cyanide-petrolatum mixture and heating. The inside surface of a contact socket (which would receive the contact pin shown) and the crimp portion of the terminal can advantageously be plated by the process of the invention. A wide variety of other surfaces can advantageously be plated by the process of the invention, for example, switch contacts and contact portions of lead frames of the types used for integrated circuits.

The practice of the invention does not require specialized or complex equipment. The heating step can in fact be carried out by simply heating the part being plated on a simple electrical heating unit. It is desirable, however, to perform the heating step by means ofa simple muffle furnace or the like having a reducing atmosphere to prevent tarnishing of the substrate. The parts need not be maintained at the elevated temperature for long periods of time in order to bring about the reduction of the salt and the deposition of the gold plating. About 30 seconds has been found to be adequate in all circumstances investigated.

Articles which are to be plated in accordance with the invention, and which are not of nickel, are advantageously provided with an underplating of nickel, about millionths of an inch or more in thickness. lt has been observed that the adherence between the deposited gold and a nickel substrate is superior to the adherence between deposited gold and other metals. This feature of the invention renders it particularly useful for plating of contact terminals and the like since it is common practice in the electrical terminal art to provide, on a copper or copper alloy terminal, a plating of nickel over the copper and a plating of gold over the gold in order to improve the corrosion resistance of the part and to achieve optimum corrosion resistance. While potassium gold cyanide is a preferred gold salt for the practice of the invention, sodium gold cyanide can also be used.

The gold salt should be thoroughly mixed with the vehicle prior to application to the surface being plated in order to achieve best results as regards rapid decomposition of the salt and uniformity of the finished plating. The particles of gold salt should therefore be fine, for example, capable of passing a 300 mesh screen and they should be mixed with the vehicle long enough to ensure complete dispersion of the particles throughout the vehicle. Fine particles are also preferred for the reason that decomposition of the salt will proceed more rapidly with fine particles than with large particles.

Following are some specific examples of gold platings in ac-v cordance with the invention which describes the precise procedures for obtaining gold platings.

EXAMPLE I EXAMPLE II The procedure of Example I was followed except that the weight ratio of K Au(CN) to petrolatum 3:5. A spot of gold plating was produced as in Example I.

EXAMPLE "I The procedure of Example I was followed except that equal parts, by weight of K Au(CN) and petrolatum were used in the mixture. A spot of gold plating was produced having a thickness of approximately 200 millionths of an inch.

Changes in construction will occur to those skilled in the art and various apparently different modifications and embodiments may be made without departing from the scope of the invention. The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only.

I claim:

1. A method of providing a gold plating on a metallic substrate comprising the steps of:

coating said substrate with a composition comprising a salt selected from the group consisting of sodium gold cyanide and potassium gold cyanide and an unctuous mediheating said substrate to .a temperature in the range of about 400 F. to 600 F. for a time period sufficient to remove said unctuous medium, decompose said salt and deposit gold from said salt on said surface.

2. A method as set forth in claim 1 wherein said substrate comprises nickel and said salt comprises potassium gold cyanide.

3. A method as set forth in claim lwherein the ratio by weight of said salt to said unctuous medium is between 1:5 and 

2. A method as set forth in claim 1 wherein said substrate comprises nickel and said salt comprises potassium gold cyanide.
 3. A method as set forth in claim 1 wherein the ratio by weight of said salt to said unctuous medium is between 1:5 and 7:5. 